Narrative:

Flight from bos to pbi. Out of bos, cleared direct sey (sandy point). After reaching sey, cleared direct emsay. After using heading select to proceed to emsay and loading/executing direct emsay in FMS, I forgot to arm navigation. This resulted in an off course deviation of approximately 6 mi, because the aircraft did not turn at emsay onto J121. My personal fix for this is to no longer use the technique I learned in training of using heading select to get the aircraft turning in proper direction, then loading FMS executing and arming navigation. I will now just use FMS and not heading select for this type of situation. It's just too easy to forget to arm the navigation after you look at the pfd and the aircraft symbol is on the magenta line.

Related reports:

Google
 

Original NASA ASRS Text

Title: AN MD88 FO FAILS TO ARM THE LNAV IN THE FMS WHEN PLANNING A RADIAL INTERCEPT OFF HTO, NY.

Narrative: FLT FROM BOS TO PBI. OUT OF BOS, CLRED DIRECT SEY (SANDY POINT). AFTER REACHING SEY, CLRED DIRECT EMSAY. AFTER USING HDG SELECT TO PROCEED TO EMSAY AND LOADING/EXECUTING DIRECT EMSAY IN FMS, I FORGOT TO ARM NAV. THIS RESULTED IN AN OFF COURSE DEV OF APPROX 6 MI, BECAUSE THE ACFT DID NOT TURN AT EMSAY ONTO J121. MY PERSONAL FIX FOR THIS IS TO NO LONGER USE THE TECHNIQUE I LEARNED IN TRAINING OF USING HDG SELECT TO GET THE ACFT TURNING IN PROPER DIRECTION, THEN LOADING FMS EXECUTING AND ARMING NAV. I WILL NOW JUST USE FMS AND NOT HDG SELECT FOR THIS TYPE OF SIT. IT'S JUST TOO EASY TO FORGET TO ARM THE NAV AFTER YOU LOOK AT THE PFD AND THE ACFT SYMBOL IS ON THE MAGENTA LINE.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.